1. Field of the Invention
The present invention relates to a crystalline polymer microporous film with a high filtration efficiency for use for microfiltration of gas, liquid, etc., a manufacturing method of the same, and a filtration filter.
2. Description of the Related Art
Microporous films have been traditionally known and are finding increasing applications for filtration filters, etc (R. Kesting, Synthetic Polymer Membrane, McGraw Hill). Examples are, for example, microporous films made of cellulose ester (see U.S. Pat. Nos. 1,421,341, 3,133,132 and 2,944,017; and Japanese Patent Application Publication (JP-B) Nos. 43-15698, 45-3313, 48-39586 and 48-40050), microporous films made of aliphatic polyamide (see U.S. Pat. Nos. 2,783,894, 3,408,315, 4,340,479, 4,340,480 and 4,450,126; Germany Patent No. 3138525; and Japanese Patent Application Laid-Open (JP-A) No. 58-37842), microporous films made of polyfluorocarbon (see U.S. Pat. Nos. 4,196,070 and 4,340,482; and JP-A Nos. 55-99934 and 58-91732), and microporous films made of polypropylene (see West Germany Patent No.3003400).
These microporous films are used for the filtration and sterilization of water for pharmaceutical preparations, production processes for pharmaceutical preparations, foods, etc., as well as washing water for electronics. In recent years, they are used in a variety of fields and so their consumption is increasing. In particular, microporous films receive attention that can capture particles with high reliability. Among them, microporous films formed of crystalline polymer offer excellent chemical resistance, and in particular, crystalline polymer microporous films made of polytetrafluoroethylene (hereinafter may be referred to as “PTFE”) offer excellent heat resistance and chemical resistance; therefore, the demand for PTFE microporous films is significantly growing.
The permeation rate of a microporous film per unit area is generally small (i.e., filtration life is short). For industrial applications, this necessitates the use of a large number of filtration units arranged in series in order to ensure an increased film area. Thus it is necessary to increase filtration life in order to reduce costs associated with filtration. As microporous films that can effectively prevent flow rate reduction due to clogging, for example, asymmetric films have been proposed in which the pore size gradually decreases from the inlet side to outlet side (see JP-B Nos. 55-6406 and 04-68966).
Also proposed are, for example, a multilayered polytetrafluoroethylene porous film consisting of a small-pore size filtration layer and a support layer with a pore size greater than that of the filtration layer (see JP-A No. 04-351645), and a porous film fabricated by applying an emulsion of polytetrafluoroethylene onto a polytetrafluoroethylene sheet and by drawing the sheet (see JP-A No. 07-292144).
Unfortunately, in JP-A Nos. 04-351645 and 07-292144, cracks or other defects are more likely occur on the resultant microporous film after coating and drying processes. Moreover, since small pore size is obtained only around the film surface, it offers limited filtration life.
Furthermore, according to JP-A Nos. 03-179038 and 03-179039, it is possible to fabricate a microporous film consisting of a small-pore size filtration layer and a large-pore size support layer, which are integrally bonded together completely, by multiple-sheet extrusion. In this microporous film, however, clogging easily occurs at the boundary between the filtration layer and support layer, i.e., at a portion where pore size change is discontinuous.
JP-B No. 63-48562 discloses a method of manufacturing a thin film with asymmetric pore size by pressing a thin ethylene tetrafluoride resin film while heating both surfaces thereof at different temperatures. With this proposal, however, pores of desired shape cannot be successfully obtained since the heating temperature is low—from 250° C. to the melting point of ethylene tetrafluoride resin.
Accordingly, the current situation is that demand is growing for a crystalline polymer microporous film consisting of two or more crystalline polymer layers, which enables efficient capturing of microparticles without clogging and thus gives longer filtration life, a manufacturing method of the crystalline polymer microporous film, and a filtration filter including the crystalline polymer microporous film.